Apparatus for the refrigerated storage of liquefied gas



y 1964 J. B. MAHER ETAL 3,132,489

APPARATUS FOR THE REFRIGERATED STORAGE 0F LIQUEFIED GAS Filed Jan. 5. 1961 INVENTORS: James B. Ma/rer BY James Ma y A Merriam, SID/fl! 8 Marshall Q A T TOR/V5 Y5 Fig. 2

.tional insulating techniques. by a primary system which maintains the stored material United States Patent 3,132,489 APPARATUS FOR THE REFRIGERATED STORAGE 0F LIQUEFIED GAS James B. Maher and James Mair, Chicago, Ill., assignors to Chicago Bridge & Iron Company, Chicago, Ill., a corporation of Illinois Filed Jan. 3, 1961, Ser. No. 80,476 5 Claims. (Cl. 62-54) This invention relates to an improved apparatus for the refrigerated storage of liquefied, normally gaseous products, such as NHg, L.P.G., propane, oxygen, etc.

Such liquid materials can he economically stored in large, refrigerated, essentially flat bottom, cylindrical storage tanks at essentially atmospheric pressure. Generally vapor-compression refrigeration systems employ a series of steps which include compressing the product vapor, condensing the compressed vapor with water or air as the condensing medium, and flashing or releasing the condensed liquid into an atmosphere of much lower pressure to maintain the subatmospheric temperature storage conditions. Because of the large pressure and temperature changes involved in refrigeration systems of this type, efficient compression tnust be achieved by multi-stage compression.

In commercial storage installations, the principal material stored contains uncondensible constituents such as air, ethylene and methane which, because they will not change state to a liquid at usual operating storage conditions of temperature and pressure, must be purged or vented to the atmosphere. If they are not, then these non-condensible constituents will accumulate and overload the compressors. Furthermore, during purging, a significant amount of the stored material is also lost to the atmosphere.

Accordingly, the instant invention provides an apparatus for the refrigerated storage of liquefied, normally gaseous materials which minimizes the purging problems, reduces the net power requirements, reduces the. number of stages for compression as well as provide other features for affecting installation and operating economies. The

system of this invention operates at relatively low temperatures and consequently, at relatively low pressure, thus minimizing the problem of purging which is more serious at high pressure operation.

Referring to the attached drawings FIGURE 1 is a schematic flow sheet showing an embodiment of the refrigerated storage system of this invention; and,

FIGURE 2 shows a preferred embodiment of a condenser and flash tank employed for heat interchange in the refrigeration system.

In the illustrative embodiment shown in FIGURE 1 a commercial propane storage system which uses a secondary refrigeration system employing substantially pure propane, i.e., containing no ethylene, ethane, or other constituents having. a lower boiling point than propane as the refrigerant is presented. Product storage in this instance is provided by one or more conventional flat .botton, single wall or double wall storage vessels 10. The

storage vessels 10 are adequately insulated using conven- Refrigeration is provided at selected storage conditions and a second system functioning as hereinafter described.

Material to be stored is received through line 11.

.The material which is received at a temperature higher than the storage temperature passes through a combination condenser and dash tank 12 where the material is "ice tom tank it The vapor resulting from the flashing is substantially completely condensed within condenserflash tank 12, admixing with the liquid flashed product and is also passed to storage. As product evaporation occurs in [the fiat bottom tank 10, the warm vapor is removed from the vapor space through line 14 and circulated through the primary refrigeration system to be used ultimately as the refrigerant therein. The vapor passes through either the large fill compressor 15 or the small hold compressor 16, depending, respectively, upon Whether or not the storage system is receiving material to be stored from an outside source through line 11 or is maintaining storage conditions for the stored material. To permit the alternative use of compressors 15 and 16, valves 17 and 18 are operated in order to direct the vaporous material through line 19 and the fill compressor 15 or through line 20 and hold compressor 16. The compressed vaporous material discharged from the compressor 15 or 16 being used is circulated through line 21 into the condenser-flash tank 12 to mix with the incoming material stream from line 11. The condenser-flash tank 12 is provided with a safety vent 25 which allows purging of the relatively small amount of non-condensibles which might accumulate there.

The secondary refrigerant circulates through the tube sideof the condenser-flash tank 12 where it picks up heat by conventional heat transfer from the vaporous material stored from the primary refrigeration system. In the heat transfer process the secondary refrigerant is vaporized. The vapor from the tube side of the condenser-flash tank 12 is then drawn 01f through line 26 and passes through hold compressor 27 where it is compressed to a substantially higher pressure and discharged through line 28 to be condensed in condenser 29. Generally, sufficient heat transfer can be effected employing air or cool water as the heat transfer medium in hold compressor condenser 29. The fill compressor 30 used during the secondary refrigerant fill cycle may require the use of a separate refrigeration unit 31 to provide antificially cooled coolant, which may be water, air or another suitable fluid, through lines 32 and 33 to condenser 34.

One of the main advantages of this system is that it operates at relatively low temperatures and consequently at relatively low pressure, thus minimizing the problem of purging which is much more serious at high pressure.

Although heat exchange between the primary refrigerant (stored material) and secondary refrigerant can be efiected in a variety of conventional heat exchange apparatus, a preferred heat exchange is shown in FIGURE 2. The illustrated combination condenser-flash tank 12 is a vertical, shell-and-tube vessel in which the secondary refrigerant is evaporated on the tube side at a low temperature by hotter circulating vapors from the primary refrigeration system which are condensing on the shell side at a higher temperature. The condenser-flash tank has a multi-functional purpose serving, in the shell side, to flash vaporize the incoming material to be stored to a low intermediate temperature to reflux and condense the vapor resulting from the flash vaporization of the stored product,

and to serve as a condenser for the compressed vapor produced in the primary refrigeration cycle. The operating conditions for the condenser-flash tank are selected to provide a balancing of the compression ratios in the compressors. The area of heating surface afforded by the tubes and the cross sectional area of the vapor release section are calculated from well-known heat transfer formulae. The body 35 of the illustrative condenser-flash tank is a vertical cylinder, closed top and bottom by dished and flanged or elliptical heads 36. The nozzles 37 and 33 are provided on the tube side, respectively, as

flanges, depending on the method of construction and materials used. These tube sheets have a plurality of openings to receive a like number of tubes 41 to provide the required heating surface; the tubes 41 project slightly beyond each tube sheet 39 and 40 and can be rolled in or held in place by other conventional techniques. The tubes 41 can be spaced on triangular pitch with sufiicient clearance to insure vapor penetration with a minimum of pressure drop. The shell side of the condenser-flash tank 12 is provided with nozzles 42, 43, 44, and 45 for use respectively as an inlet for compressed vaporous stored product connected to line 21, a liquid material inlet connected to line 10, a liquid material outlet connected to line 13 and a purge vent gas outlet connected to line 25. The unit can be provided with legs to rest on supporting steel, or other suitable foundations, or can be self-supporting with columns resting on grade.

In using the condenser-flash tank above described in the refrigerated storage system of this invention, the liquid secondary refrigerant, entering the bottom of condenser-flash tank 12, flashes and is boiled by the condensing vapor on the shell side of the tubes. The turbulence and velocity of liquid rising in the tubes is conducive to the maintenance of high rates of heat transfer. The liquid refrigerant level is maintained, by suitable liquid level controls, not shown, in the vapor-liquid disengaging space, just above the top tube sheet 39.

Although flat-bottom storage vessels are preferably used in storage of the material, other types of storage tanks such as spherical, dished bottom cylindrical, and others can be used depending upon selected storage conditions of temperature and pressure or customer selection. Adequate insulation, however, must be used regardless of the type of tank selected. The storage facility can be equipped with conventional vapor conservation systems 46 where inert gases are used in double wall tank insulation systems. Also a product heater 47, steam heated as shown or otherwise heated, is used to heat the material leaving the storage facility for distribution purposes. The refrigeration systems used for the primary and secondary systems are preferably vapor compression systems as illustrated; however, other types of refrigeration systems can also be used. For example, in an alternate primary refrigeration system vapor from the storage vessel is removed through line 14 and line 48 (shown dotted) by means of pump 49 (shown dotted) and passed to condenser-flash tank 12 via line 21 where it is condensed The resulting liquid returns to storage tank 10 through line 13. With this arrangement the compressors 15 and 16 are not used. The refrigerant selected for the secondary system depends upon the characteristics of the stored material and so-called cascade cooling systems can be used in the secondary refrigeration system. In Table I is tabulated several materials which can be stored in the system of this invention and secondary refrigerants which can be used.

Table I Stored Product (Primary Secondary Refrigerant Refrigerant) Anhydrous Ammonia or Freon.

Pure Propane (substantially free from Ethylene, eto.).

Pure Ammonia, Freon or Pure Commercial Butanes Commercial Propane Commercial Anhydrous Ammonia in successive cascade arrangement.

In one specific installation commercial liquid propane is received at the rate of 5,000 long tons per month (15,556 pounds per hour) at 90 F. and using the system of this invention is cooled sufficiently to reduce the vapor pressure to a storage condition of near atmospheric pressure (a maximum ambient temperature at plant site of F.). Product storage is provided in four (4) 100,000 barrel flat bottom, dome roof, double wall tanks. The annular space between the inner storage vessel and outer tank shell and roof is filled with granular perlite insulation. Air is initially removed from this space by purging with nitrogen. After the space is filled with perlite and nitrogen, a slight positive pressure (2 to 4 inches of water) is maintained. To hold this constant pressure during changes in ambient temperature and barometric pressure, the space breathes nitrogen to .or from a suitable vapor tank.

In the primary refri eration cycle, vapor at about 15.2 p.s.i.a. is drawn off the storage tank, compressed in single stage to 55.2 p.s.i.a. and condensed, at this low pressure and corresponding temperature, by an independent boiling secondary refrigerant. The condensed liquid is flashed back into the storage tank thus completing the conventional vapor compression cycle. The 90 F. incoming stream is first flashed also to 55.2 p.s.i.a. in the shell side of the combination vessel. The resulting vapor is immediately condensed by the independent refrigerant and this condensate, along with the residual incoming liquid, both at the lower temperature and pressure, are discharged to the storage tank. The vapor flashed, as this liquid enters the 15.2 p.s.i.a. storage tank, is withdrawn by the compressors and goes through the cycle described above. The shell side of the combination vessel serves as both the condenser and the flash tank. The condensing medium, the independent boiling refrigerant, is contained in the tube side of this vessel. The tube side also functions as the evaporator for the secondary refrigerant cycle.

In the secondary refrigerating system, pure propane is used as the refrigerant. Vapor is Withdrawn from the evaporator portion of the combination vessel, compressed in single stage to about 200 p.s.i.a. and then condensed in a shell and tube condenser with Water as the condensing medium. The condensed refrigerant is flashed back to the evaporator, completing the conventional closed vapor compression cycle.

The combination vessel is provided with a refrigerated purger on the shell side to remove non-condensables from compressed or flashed vapors.

Most commercial L.P.G. products contain some lower boiling point components such as ethane. At the intended storage temperature and near atmospheric pressure, such lighter gases must be considered as non-condensables. In the cycle described above, the non-condensable gases in the vapor mixture are withdrawn from the storage tank by the compressors. They may be purged conveniently and safely from the condenser section of the combination vessel.

The vapor compression cycle provides sensitive and positive control of storage tank pressure. The storage tank receives only liquid flashed from a low temperature. The percentage of vapor resulting from the final flash is small in comparison to the quantity of vapor which would be formed in flashing the fill stream directly from 90 F. to a storage pressure of near atmospheric. The lesser required vapor flow, the more positive control of non-condensables, and the considerable savings in required horsepower all contribute to the overall advantages of this cascade type refrigeration system.

At maximum ambient conditions, the holding refrigeration requirement for the four tanks is 28 tons. The filling compressor will act as the auxiliary holding compressor thus providing percent stand by refrigeration for holding. The pure propane refrigeration system has a compressor and condenser which is of sufficient capacity to maintain the tubes in the condenser flash tank at 5 F. during maximum filling and holding refrigeration requirements. Stand-by can also be provided for holding refrigeration in this system.

A steam to product heat exchanger is included to heat the stored product to 40 F. before it enters the distribution system. This unit includes an automatic temperature regulator and discharge trap.

The instant invention has particular application in the storage systems having capacities of 10,000 to 600,000 barrels; it is, however, adapted to other size installations for the storage of ammonia, C C hydrocarbons, especially C and C hydrocarbons. For simplicity purposes expansion valves and other flow control devices, bypasses, pumps, entrainment separators, instrumentation related to the subject invention have been omitted from the foregoing description. Although the invention has been described with reference to a specific embodiment of a refrigerated storage installation, various modifications and variations can be effected by those skilled in the art without departing from the scope of this invention. Accordingly, the above description is given for clearness of understanding of this invention which is defined in the appended claims.

We claim:

1. A refrigerated storage system for storing a liquefied normally gaseous material which comprises an insulated storage vessel; a primary refrigeration system including conduit means for transferring vaporous material from said vessel, means for cooling and liquefying said vaporous material including indirect heat exchange flash-condenser means having a shell side and a tube side, and means for connecting said conduit means to one side of said heat exchange means; and a secondary refrigeration system for supplying a coolant connected to the other side of said heat exchange means; a second conduit means for supplying material to be stored, means for connecting said second conduit means to said one side of said flash-condenser means; and means for withdrawing liquid product from said one side and passing it to said storage vessel.

2. A refrigerated storage system for storing a liquefied normally gaseous material which comprises an insulated storage vessel; a primary vapor compression refrigeration system including conduit means for transferring vaporous material from said ,vessel, compressor means for compressing vaporous material removed from said vessel, indirect heat exchange flash-condenser means having a shell side and a tube side, and means connecting the discharge side of said compressor to one side of said heat exchange means; a secondary refrigeration system for supplying a coolant connected to the other side of said heat exchange means; a second conduit means for supplying liquid material to be stored, means for connecting said conduit means to said one side of said flash-condenser means; and means for withdrawing liquid product from said one side and passing it to said storage vessel.

3. A refrigerated storage system for storing a liquefied normally gaseous material which comprises an insulated storage vessel; a primary vapor compression refrigeration system including conduit means for transferring vaporous material from said vessel, compressor means for compressing vaporous material removed from said vessel, indirect heat exchange fiash tank-condenser means having a shell side and a tube side, and means connecting the discharge side of said compressor to one side of said heat exchange means; a secondary refrigeration system for supplying a coolant connected to the other side of said heat exchange means; means for flash vaporizing material to be stored into said one side of said heat exchange means; and means for withdrawing liquid product from said one side and passing it to said storage vessel.

4. A refrigerated storage system for storing a liquefied normally gaseous material which comprises an insulated storage vessel; a primary vapor compression refrigeration system including conduit means for transferring vaporous material from said vessel, compressor means for compressing vaporous material removed from said vessel, in direct heat exchange flash-condenser means having a shell side and a tube side, and means connecting the discharge side of said compressor to one side of said heat exchange means; a secondary vapor-compression refrigeration system for supplying a coolant connected to the other side of said heat exchange means; a second conduit means for supplying liquid material to be stored, means for connecting said second conduit means to said one side of said flash-condenser means; and means for withdrawing liquid product from said one side and passing it to said storage vessel.

5. A refrigerated storage system for storing a liquefied normally gaseous material which comprises an insulated storage vessel; a primary vapor compression refrigeration system including conduit means for transferring vaporous material from said vessel, compressor means for compressing vaporous material removed from said vessel, indirect heat exchange flash tank-condenser means having a shell side and a tube side, and means connecting the discharge side of said compressor to said shell side of said heat exchange means; a secondary vapor-compression refrigeration system for supplying a coolant connected to said tube side of said heat exchange means; means for flash vaporizing product to be stored into said shell side of said heat exchange means; and means for withdrawing liquid product from said shell side and passing it to said storage vessel.

References Cited in the file of this patent UNITED STATES PATENTS 1,274,479 Barth Apr. 30, 1918 2,321,445 Yendall et al. June 8, 1943 2,783,624 Morrison Mar. 5, 1957 2,944,406 Anderson July 12, 1960 

1. A REFRIGERATED STORAGE SYSTEM FOR STORING A LIQUEFIED NORMALLY GASEOUS MATERIAL WHICH COMPRISES AN INSULATED STORAGE VESSEL; A PRIMARY REFRIGERATION SYTEM INCLUDING CONDUIT MEANS FOR TRANSFERRING VAPOROUS MATERIAL FROM SAID VESSEL, MEANS FOR COOLING AND LIQUEFYING SAID VAPOROUS MATERIAL INCLUDING INDIRECT HEAT EXCHANGE FLASH-CONDENSER MEANS HAVING A SHELL SIDE AND A TUBE SIDE, AND MEANS FOR CONNECTING SAID CONDUIT MEANS TO ONE SIDE OF SAID HEAT EXCHANGE MEANS; AND A SECONDARY REFRIGERATION SYSTEM FOR SUPPLYING A COOLANT CONNECTED TO THE OTHER SIDE OF SAID HEAT EXCHANGE MEANS; A SECOND CONDUIT MEANS FOR SUPPLYING MATERIAL TO BE STORED, MEANS FOR CONNECTING SAID SECOND CONDUIT MEANS TO SAID ONE SIDE OF SAID FLASH-CONDENSER MEANS; AND MEANS FOR WITHDRAWING LIQUID PRODUCT FROM SAID ONE SIDE AND PASSING IT TO SAID STORAGE VESSEL. 